Could you choose the most natural sentence, please?
- As a kitten, she fell from an attic of a nine-storey bulding into a ventilation shaft.
- As a kitten, she fell from the attic of a nine-storey bulding into a ventilation shaft.
Attic: space within the roof of a house
So, clearly only one attic is possible in a house. Same is with a building (with apartments), as there is only one roof. Thus, the more natural sounding (and correct) alternative is:
It is possible to have more than one attic in a building if a wall extends through the roof-space to the roof — it's often called a firewall and its purpose is to stop fire spreading through the entire roof-space.
So an attic is entirely correct, if slightly unusual because most buildings aren't large enough to require firewalls to create separate attics. Perhaps a nine-storey building would be large enough.
As for the "most natural" sentence, it probably is the attic, simply because most buildings have only one.